Cascaded PWM Converter Control for Low-Loss Harmonic Reduction

Solution Overview

Problem

Medium voltage power converters operate at low switching frequencies due to high switching losses in Si-based switches, and while SiC-based switches offer potential for higher frequencies, they are more expensive and not widely adopted.

Innovation Solution

A method for operating an electrical converter that includes a main converter and multiple converter cells, where the main converter generates an output voltage and subsequent cells further process this voltage using pulse width modulation with increasing frequencies to minimize voltage errors and reduce harmonics, allowing for the use of SiC switches with lower switching losses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Loss of energy

If Si-based switches (IGBTs/IGCTs) are used in medium voltage converters, then the converter can operate reliably, but switching losses are high and switching frequency is limited to a few hundred Hertz

Engineering Contradiction:

Improveswitching lossesVSAvoidswitching frequency

Core Design Contradiction:

Loss of energyVSSpeed

Solution Approach 1:

The converter is divided into multiple independent converter cells (first converter cell, second converter cell, etc.), each operating at different switching frequencies. The first converter cell operates at lower frequency with Si-based switches, while subsequent cells operate at higher frequencies with SiC-based switches, allowing the system to achieve high effective switching frequency without requiring all components to operate at high frequency.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different semiconductor materials are used in different parts of the system: Si-based switches are used in the first converter cell where lower switching frequency is acceptable, while SiC-based switches are used in subsequent converter cells where high switching frequency is needed to reduce losses and filter requirements.

Inventive Principle:

Principle #3Local quality

2Loss of energy

If SiC-based switches are used to achieve high switching frequencies, then switching losses are reduced and switching frequency can be increased, but the cost of the converter increases

Engineering Contradiction:

Improveswitching lossesVSAvoidconverter cost

Core Design Contradiction:

Loss of energyVSEase of manufacture

Solution Approach 1:

The converter is divided into multiple independent converter cells (first converter cell, second converter cell, etc.), each operating at different switching frequencies. The first converter cell operates at lower frequency with Si-based switches, while subsequent cells operate at higher frequencies with SiC-based switches, allowing the system to achieve high effective switching frequency without requiring all components to operate at high frequency.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different semiconductor materials are used in different parts of the system: Si-based switches are used in the first converter cell where lower switching frequency is acceptable, while SiC-based switches are used in subsequent converter cells where high switching frequency is needed to reduce losses and filter requirements.

Inventive Principle:

Principle #3Local quality

3Loss of energy

If a single converter operates at high switching frequency to reduce losses, then switching losses are reduced, but large passive filters are still required and harmonic distortion remains

Engineering Contradiction:

Improveswitching lossesVSAvoidharmonic distortion

Core Design Contradiction:

Loss of energyVSObject-generated harmful factors

Solution Approach 1:

The converter is divided into multiple independent converter cells (first converter cell, second converter cell, etc.), each operating at different switching frequencies. The first converter cell operates at lower frequency with Si-based switches, while subsequent cells operate at higher frequencies with SiC-based switches, allowing the system to achieve high effective switching frequency without requiring all components to operate at high frequency.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Multiple converter cells operate at different periodic switching frequencies, creating a distributed switching pattern. This periodic action at multiple frequency levels helps to spread and reduce harmonic distortion across the frequency spectrum, eliminating the need for large passive filters while maintaining low losses.

Inventive Principle:

Principle #19Periodic action

Data Source

PatentUS11784583B2Cascaded pulse width modulation converter control

Publication Date: 2023.10.10 ABB (SCHWEIZ) AG

AI summary

An electrical converter includes a main converter for generating a first output voltage and a converter cell for converting the first output voltage into a second output voltage. A method for operating an electrical converter includes: receiving a reference voltage for the electrical converter; pulse width modulating the reference voltage with a first modulation frequency for generating a first switching signal for the main converter; switching the main converter with the first switching signal to generate the first output voltage; estimating the first output voltage from the first switching signal; determining a voltage error by subtracting the estimated first output voltage from the reference voltage; pulse width modulating the voltage error with a second modulation frequency, which is higher than the first modulation frequency, for generating a further switching signal for the converter cell; and switching the converter cell with the further switching signal to generate the second output voltage.